Carbohydrates are ubiquitous in biological systems, involved in such important functions as inflammation (Phillips, M. L. et al. Science 1990, 250, 1130; Lasky, L. A. Science 1992, 258, 964; Giannis, A. Angew. Chem. Int. Ed. Engl. 1994, 33, 178; and Yuen, C.-T. et al. J. Biol. Chem. 1994, 269, 1595), immunological response (Varski, A. Proc. Natl. Acad. Sci. USA 1994, 91, 7390; Ryan, C. A. Proc. Natl. Acad. Sci. USA 1994, 91, 1; and Meldal, M. et al. in Carbohydrate Antigens; (Garegg, P. J. et al., Eds; ACS Symposium Series No. 519; American Chemical Society; Washington, D.C., 1993)), metastasis (Feizi, T. Curr. Opin. Struct. Biol. 1993, 3, 701), and bacterial and viral infection (Varski, A. Glycobiology 1993, 3, 97). While the synthesis of peptides and oligonucleotides was automated decades ago, there are no such general synthetic procedures available for the construction of complex oligosaccharides. Several recent reviews on oligosaccharide synthesis have been published (Paulsen, H. Angew. Chem. Int. Ed. Engl., 1990, 29, 823–839; Banoub, J. Chem Rev. 1992, 92, 1167–1195; Toshima, K. et al. Chem. Rev. 1993, 93, 1503; Schmidt. R. R. et al. Adv. Carbohydr. Chem. Biochem. 1994, 50, 21; and Danishefsky, S. J. et al. Angew. Chem. Int. Ed. Engl., 1996, 35, 1380). The necessity for regio- and stereo-control in glycoside bond forming processes often leads to laborious synthetic transformations, tremendous protecting group manipulations, and tedious intermediate isolations which complicate the overall synthetic process and decrease synthetic efficiency.
In order to facilitate the rapid synthesis of oligosaccharides, a new chemoselective glycosylation strategy, the “one-pot sequential glycosylation,” has recently been developed (Fraser-Reid, B. et al. C. Synlett 1992, 927; Raghavan, S. et al. J. Am. Chem. Soc., 1993, 115, 1580; Yamada, H. et al. Tetrahedron Lett., 1994, 35, 3979; Yamada, H. et al. J. Am. Chem. Soc. 1994, 116, 7919; Chenault, H. K. et al. Tetrahedron Lett., 1994, 35, 9145; Ley, S. V. et al. Angew. Chem. Int. Ed. Engl. 1994, 33, 2292; Grice, P. et al. Synlett 1995, 781; Geurtsen, R. et al. J. Org. Chem. 1997, 62, 8145; Grice, P. et al. Chem. Eur. J. 1997, 431). This approach is based on the observation that a large disparity between the reactivities of different glycosyl donors can be achieved simply by varying the protecting groups and the electron donating or withdrawing character of the leaving group within a given class of glycosyl donors (e.g. thioglycosides). As a result, even though identical chemistry is performed at each glycosidic coupling step, with proper planning a high degree of sequence selectivity can be achieved between competing donors of the same class, eliminating the need for protecting group manipulation between coupling steps. The synthetic approach is designed such that the choice of protecting groups on sugar components (ibid, Fraser-Reid, B. et al. 1992; Raghavan, S. et al. 1993; and Yamada, H. et al. 1994), or the combination of protecting groups and anomeric substituent (ibid, Yamada, H. et al. 1994; and Chenault, H. K. et al. 1994) will lead to a decrease in donor reactivity over the course of the synthetic sequence. The most reactive donor is used for the non-reducing end and an unreactive donor is used for the reducing end of the given oligosaccharide target.
Using these procedures, multiple coupling steps were successfully performed by many laboratories to generate various lengths of complex oligosaccharides.(Green, L. et al. Synlett. 1998, 4, 440). This methodology is, however, not generally applicable because of the lack of precise reactivity values of useful glycosyl donors and acceptors. Quantitative analysis of the glycosylation reactivity of several glycosyl donors can been achieved using NMR (Douglas, N. L. et al. J. Chem. Soc. Perkin Trans. 1, 1998, 51). A particularly desirable goal in this research is to establish a generally applicable method that will allow the rapid synthesis of desired oligosaccharides from designed monomeric building blocks in a programmable and predictive manner. Such a method can be used in the rapid assembly of complex oligosaccharides and may be further developed toward automation.